1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Related Background Art
Conventionally, it has been actively studied to multiplex, in image information, other information concerning an image represented by the image information.
In recent years, a technique so-called an electronic watermark technique is standardized. The electronic watermark technique is the technique to multiplex, in the image information representing a photograph, a painting and the like, additional information representing its author's name, license and the like so that it becomes difficult to visually distinguish the additional information, and to circulate the multiplexed information through a network such as the Internet or the like.
As general methods of embedding the additional information, the electronic watermark technique provides two methods, that is, one of embedding the additional information in real space, and the other of embedding the additional information by using a frequency band.
The simplest method of embedding the electronic watermark in real space is the method of decomposing the image information into bit planes, and allocating the bit plane of LSB (least significant bit) as the bit plane of the additional information.
Besides, in the method of embedding the additional information by using the frequency band, orthogonal transformation such as Fourier transformation, discrete cosine transformation, wavelet transformation or the like is used. In any case, there is the method of embedding the additional information by changing a transformation coefficient of a specific band with use of a high frequency band which is difficult to be visually remarkable or low and middle frequency bands which are difficult to be influenced by quantization of the transformation coefficient in data compression. Moreover, there is the electronic watermark method that resistance to attack is hardened by using spectrum diffusion.
These conventional methods as above are explained in detail in Kineo MATSUI “Basic of Electronic Watermark”, Morikita Publishing Company, August 1998.
However, there are following problems in these conventional methods.
That is, the conventional method of embedding the electronic watermark premises that it is used on an electronic file.
FIG. 11 is a diagram showing additional information embedding which is generally performed in the electronic watermark technique, that is, image information A and additional information B are multiplexed by an addition unit 1101 to generated multiplexed information C. Incidentally, FIG. 11 shows an example that the additional information is multiplexed in the real space region of the image information. If it is possible to circulate the multiplexed information C without various image processes such as filtering and the like and various encoding such as unreversible compression and the like, the additional information B can be easily decoded from the multiplexed information C even in the conventional technique. If the image information circulated on the Internet has a certain level of noise resistance, the additional information B can be decoded even if digital filtering to improve image quality such as for edge emphasis, smoothing and the like is performed.
However, it is assumed that the multiplexed image information is printed by an output apparatus such as a printer or the like and the additional information is captured from such a printed image, and moreover, it is assumed that print output is performed by using the printer merely having expressive power of about two to several gradations for each color. In recent years, although an ink-jet printer which can express several gradations for each color by using ink of low-density dye and variably controlling output dot diameters is available, the gradation of a photographic image can not be expressed as long as a pseudo-gradation process is not performed.
That is, on the premise that the multiplexing method using the electronic watermark technique shown in FIG. 11 is applied to the printer output, as shown in FIG. 12, the multiplexed information C is converted into quantized information D through a pseudo-gradation process 1201 and then printed on a paper through printer output 1202, whereby the multiplexed information C is changed to on-paper information (printed image) E having deteriorated image quality. Therefore, to decode the additional information from the on-paper information is to decode the additional information B from the on-paper information E after a series of processes in FIG. 12 is performed. Incidentally, information change amounts by the processes 1201 and 1202 are remarkably large, whereby it is very difficult to multiplex the additional information so that the multiplexed information can not be visually discriminated and to correctly decode the multiplexed additional information from the paper.
In the above method of embedding the additional information in the bit plane in real space, it is almost impossible to decode the embedded information on the paper.
On one hand, as for this, even a method of changing a power value in a specific frequency band is similar.
For example, Japanese Patent Application Laid-Open No. 7-123244 proposes a technique of multiplexing information by embedding additional information in a color difference component of which sensitivity is visually low and a high frequency band of a saturation component.
However, in this technique, each bit data is allocated to each specific band (or specific frequency band), and “1” or “0” is expressed according to whether or not a pattern should be added to the specific band. Besides, in decoding, a frequency component of the specific band is extracted, and “1” or “0” is discriminated according to a threshold process.
In this method, there are the following problems.
Since the threshold process is performed to the absolute value of the power, the decoding from the paper is easily influenced by noise.
In order to strengthen against the noise, it is necessary to perform the multiplexing strongly, whereby image quality deteriorates.
Since a pseudo-gradation process is performed at a later stage, there is a fear that the absolute value of the power deteriorates due to filtering of the pseudo-gradation process.
In order to cause the paper to have resistance to rotation, a start bit is necessary in the specific band.
The process is complicated as a whole.
That is, as above, the method of discriminating “1” bit or “0” bit according to the magnification of the absolute value of the power in the specific band has many problems.